The invention relates to a vacuum brake booster comprising a vacuum chamber and a working chamber separated from each other by a movable wall, a control valve which comprises a housing coupled workingly to said movable wall and which is capable of controlling the supply of atmospheric pressure or above-atmospheric to the working chamber in dependence upon the displacement of an input element of the brake booster to achieve a pressure difference at the movable wall, a sensing disk disposed in an axially displaceable manner inside the control valve housing, and a coupling device which introduces forces acting counter to the actuating direction upon the sensing disk in a coupled position into the control valve housing and in an uncoupled position via an actuating piston into the input element, wherein the coupling device comprises a coupling element for introducing the forces acting counter to actuating direction into the control valve housing as well as an axially displaceable actuating element.
Vacuum brake boosters have been known for quite some time and millions of them are being used to boost the actuating forces of a vehicle hydraulic brake system and therefore keep said forces at a level which is acceptable to the driver of a vehicle. Likewise known are so-called emergency braking aids, which are frequently also referred to as “brake assistants”. These are devices which provide a driver in an emergency braking situation with increased braking power for substantially the same actuating force.
Emergency braking aids may be divided into electro-magnetically actuated and mechanically actuated systems. For reasons of cost, the use of a mechanical system is preferred for applications in vehicles at the lower end of the price range.
A vacuum brake booster having such a mechanical emergency braking aid is known, for example, from DE 198 31 962 A1. Said vacuum brake booster has a vacuum chamber and a working chamber separated from each other in a pressure-proof manner by a movable wall. A control valve, which has a housing workingly coupled to the movable wall, comprises an atmospheric valve seat which, to achieve a pressure difference at the movable wall, is capable of controlling the supply of atmospheric pressure to the working chamber in dependence upon the displacement of an input element of the brake booster.
The input element is coupled in actuating direction to an actuating piston. In the context of the present invention, the term “actuating direction” is always the actuating direction of the brake booster. Disposed in actuating direction downstream of the actuating piston is a sensing disk, which cooperates with the reaction disk of a power delivery element. Said axially displaceable sensing disk allows the transmission of a load from and to the actuating piston.
For improved boosting of the braking force in emergency braking situations, a mechanical emergency braking aid is disposed in the control valve housing. The emergency braking aid comprises a coupling device which introduces reaction forces of the master brake cylinder, which act counter to actuating direction upon the sensing disk, in a coupled position into the control valve housing and in an uncoupled position via the actuating piston into the input element. In an emergency braking situation the coupling device adopts the coupling position so that the reaction forces of the master brake cylinder are introduced into the control valve housing and the holding load, which is to be summoned up by the driver and is introduced via the input element into the brake booster, is reduced by the reaction forces. To maintain the brake pressure, the driver therefore has to summon up only a relatively low actuating force.
The coupling device according to DE 198 31 961 A1 comprises a coupling element having two halves, which are displaceable in a radially outward direction and provided with sloping surfaces and which cooperate with corresponding sloping surfaces of an axially displaceable actuating element in order to effect radially outward displacement of the coupling element halves. In coupled state, the displaced coupling element halves engage behind a stop of the control valve housing so that reaction forces acting counter to actuating direction upon the sensing disk are transmitted from the sensing disk to the coupling element halves and from the latter to the control valve housing.